Документ взят из кэша поисковой машины. Адрес оригинального документа : http://xmm.vilspa.esa.es/sas/8.0.0/doc/ommosaic/node5.html
Дата изменения: Wed Oct 22 18:03:35 2008
Дата индексирования: Tue Aug 18 11:59:53 2009
Кодировка:
Поисковые слова: ngc 7023

XMM-Newton Science Analysis System


ommosaic (ommosaic-1.18.1) [???]

Meta Index / Home Page

Description

This task accepts an arbitrary number of PPS OSW rotated image files and produces a mosaiced image. The task description is basically the same as that for emosaic, from which it has been derived, with the important difference that it does not need an exposure image as ommosaic uses the exposure time keywords from the input file headers to build the exposure map for the combined image.

The mosaiced image is not intended to be used for accurate photometry as the exposure time might vary for different parts of the image. Also, the photometric information could be partially lost in the rotated images supplied to the input of ommosaic.

The task can process a mixture of both low and high-resolution images, in which case a warning is issued and the pixel-size of the mosaiced image is that of the low-resolution images (this is yet another reason against using the mosaiced image for photometric purposes). The output image is normalised to the exposure time defined by the input exposure parameter (default 1000 seconds).

The output pixel size (in degrees) and world-coordinate system (WCS) reference coordinate for the output image are set to those of the first image with the largest pixel size. The task can read 16-bit, 32-bit or 64-bit input images, the output image is (currently) always of 32-bit real type.

In the case of astrometry-corrected images the task uses the information from the CRVAL1 and CRVAL2 keywords to shift individual images with respect to the reference position of the combined image in order to get the overlapping parts of the images fit each other and to get the correct sky-projection of the combined image. The reference position of the combined image corresponds to the average values of the CRVAL1 and CRVAL2 keywords for those input images, whose POSCOROK keyword is true (this keyword is set to true by the omatt task in the case of the successful astrometric correction of the image, otherwise this keyword is set to false). If none of the input images were astrometrically corrected then the CRVAL1 and CRVAL1 keywords of the combined image are set to the corresponding values of the first image with the largest pixel size.

If some of the POSCOROK keyword is false for some of the exposures then the task will try to interpolate the RA and DEC offsets taken from two nearest in time exposures. The offsets are recorded by the omatt task in the RA_OFF and DEC_OFF attributes of the image header. If interpolation is impossible (for instance, when there are no available offsets before and after the time when the not astrometrically corrected exposure was taken) then the offsets from the nearest in time exposure with POSCOOK=true is used to adjust the position of the not astrometrically corrected exposure in the combined output image.